I run stock Android and the disconnect of headphones or bluetooth promptly pauses pandora, google music and pocket casts.
Google music does have the habit starting music playback when connecting to bluetooth regardless if the app is running or it's been a week since connecting to bluetooth or the phone has been rebooted since connecting. It just spams my car stereo with music AFAICT.
Mine's not quite stock Android, so whenever something doesn't work right I'm never sure whether it's Google's problem or Sony's.
Another audio bug: when Google Maps gives audio directions, it pauses my podcasts. Half the time it resumes playing afterward, half the time it doesn't. I can find no pattern to this.
PocketCasts and Google Maps are both pretty well polished programs, and I have no idea whether I should blame one of them or blame the OS. Ah well. At least I got an SD card slot instead of paying $200 for 48GB more storage.
I've been using Pocket Casts and Google Navigation together for a while. There is a setting in pocket casts to play over notifications and navigation prompts. I had to switch to this a few months ago because navigation prompts started getting muted for about 1 second, then they became audible half-way through and then the transition back to the podcast worked fine.
Many car stereos actually send the play signal when they connect to a device. My Alpine unit did this, as does my Pioneer AVH-4100NEX. Also guilty: 2014 Ford Focus ST and Ford Fiesta ST I tried, as well as a late model Mercedes ML500.
Google music does have the habit starting music playback when connecting to bluetooth regardless if the app is running or it's been a week since connecting to bluetooth or the phone has been rebooted since connecting. It just spams my car stereo with music AFAICT.